In this study, patients with HIV infection who are at risk for pneumocystis pneumonia based on CD4+ T cell count or prior episode of PCP have been treated orally with weekly dapsone alone or weekly dapsone plus pyrimethamine. Initially, the maximum tolerated dose of weekly dapsone as evaluated and determined to be 200 mg/weekly the addition of pyrimethamine 25 mg weekly did not appear to add to toxicity. These two combinations have been well tolerated and safe over prolonged dosing. Patients have been followed for the development of pneumocystis pneumonia, and breakthrough episodes have been observed in both arms of the study. Thus, current plans include further pharmacokinetics analysis and evaluation of the safety and tolerance of dapsone 200 mg weekly in combination with pyrimethamine 75 mg weekly. This regimen is inexpensive and may be effective prophylaxis against pneumocystis pneumonia and toxoplasma encephalitis, two important opportunistic infections in AIDS.